1. Field of the Invention
The present invention pertains to a method and an apparatus for generating a halftone image based on an image signal representing an original contone image.
2. Description of the Related Art
In color offset printing, an original contone image, such as a photograph image, is divided into four color separation images corresponding to four color inks, that is, yellow, magenta, cyan, and black. A print is reproduced by overprinting the four color separation images with respective color inks one upon another.
Usually, the four color separation images are recorded on respective photosensitive films, and four printing plates are produced from those films. An image on a photosensitive film or on a printing plate will be hereinafter referred to as "halftone image". A halftone image is composed of small dots, or halftone dots, on which the color ink is to be applied. The halftone dots are arranged at lattice points of regular intervals, and the density of the halftone image is defined by an area rate of the halftone dots. The area rate of the halftone dots is generally called a dot percent, or a halftone dot area rate.
The interval between halftone dots is defined by a screen ruling, and an orientation of the array of the halftone dots is defined by a screen angle. The screen ruling indicates the number of halftone dots per inch. The greater screen ruling reproduces an image of higher resolution. A conventional method utilizes halftone dots having a relatively low resolving power, whose screen ruling is up to about 175 lines per inch.
Recent advancement in printing technology allows applications of halftone dots having a high resolving power whose screen ruling is as much as 300 or more lines per inch. Those halftone dots will be hereinafter referred to as high definition dots. The greater screen ruling makes each dot smaller and allows an original image to be reproduced with high resolution.
The above halftone dots are arranged in a fixed array and the size of the dots is changed according to the density of the original image. In other words, an image is reproduced with halftone dots by expressing the density of the image by means of Amplitude Modulation. The density of an image can be also expressed by means of Frequency Modulation, which is called FM screening or FM dots. In the FM screening, dots on which ink is applied have a fixed size while the frequency of appearance of dots is varied with the density of the image. The FM screening uses dots which are significantly smaller than conventional halftone dots, thus reproducing an original image with high resolution.
Since the FM dots have a non-periodical arrangement unlike the conventional halftone dots, it is not correct to call the FM dots "halftone dots". In this specification, both the halftone dots and the FM dots will be called "printing dots" or simply "dots". "High resolution dots" include the FM dots and the high definition halftone dots.
The high resolution dots reproduce details of an original image more clearly and precisely than low resolution halftone dots. The high definition halftone dots having the small printing-dot size makes rosette moires sufficiently inconspicuous. The FM screening also prevents rosette moires because the arrangement of the FM dots is not periodical.
Since the high resolution dots have a small dot size, they have significantly different dot gain characteristics from those of the low resolution dots. The dot gain characteristics denotes variation in dot percent in making a printing plate from a halftone film or in actual printing procedure. The dot gain characteristics depend on a dot shape and a screen ruling. The high definition halftone dots and the FM dots have different dot gain characteristics. Application of the high resolution dots to highlight areas in an image causes a dot loss and reduces the dot percent compared with the low resolution halftone dots, whereas application of the high resolution dots to middle-tone areas and shadow areas results in a dot gain and makes the dot percent greater than that of the low resolution halftone dots.
A prepress process and a printing process are executed generally based on the dot gain characteristics of the conventional low resolution halftone dots. The low resolution halftone dots are preferable to the high resolution dots in terms of reproducibility of dot size. It may be accordingly preferred to apply high resolution dots to some areas in an original image and low resolution dots to the other areas.
If a halftone image is prepared by selectively applying high resolution dots and low resolution dots to a plurality of image areas in one image, the difference in the dot gain characteristics may cause the plurality of image area to have different printing characteristics, that is, relation between the density of a reproduced image and a level of an original image signal. Such a problem arises when a plural types of dots having different dot gain characteristics are applied to one image.
Even when each whole image is reproduced with one type of dots, two images reproduced with the same machine may have different printing characteristics if different types of dots are applied to them.